Spunbonded sheets of flash-spun polyolefin plexifilamentary film-fibril strands have been used in the past as disposable industrial garments. Such sheets have been made commercially by E.I. du Pont de Nemours & Company and sold under the trademark "TYVEK" spunbonded olefin. The sheets are known for their strength, durability, opacity and ability to act as a barrier to particulate matter that is sub-micron in size. Because of these desirable characteristics, the spunbonded sheets have been fashioned into many types of industrial garments, such as those worn by asbestos workers and disclosed in "Protective Apparel of Du Pont "TYVEK"--SAFETY YOU CAN WEAR", E-02145, (1987).
Although these sheets are useful in garment and apparel applications, there has been a need to add bulkiness and permeability to the sheets so that they are also useful in other specific end-use applications (e.g., as microfiltration fabrics). Although unconsolidated flash spun sheets have fairly high permeability in the unfused state, it is lost when the sheet is thermally bonded since fusing reduces permeability to a level that is unsuitable for PG,4 sensitive filtration applications (e.g., vacuum cleaner bags). This is attributed to high mechanical consolidation of the microdenier, ribbon-like, high surface area fibers produced by the flash spinning process. Because of the limited ability of the flash spinning and thermal bonding process to increase permeability beyond a Gurley-Hill porosity of about 8 second/100 cc, there is a significant need for a method to be developed which will increase or even maintain the permeability to a Frazier porosity of at least 4 ft.sup.3 /ft.sup.2 /min following thermal bonding.
Various methods have been suggested in the past for providing filtration media from sheets made of flash spun plexifilaments. An example of such filtration media is commercially available from E.I. du Pont de Nemours and Company under the trademark "HYSURF" filter media. However, these methods (e.g., U.S. Pat. No. 5,047,121 (Kochar)) require numerous process steps and equipment for cutting up and refining the flash spun sheets into pulp suitable for wet-laying on continuous papermaking machines. Only if these steps are carefully followed will subsequent thermal bonding produce a high grade paper suitable for microfiltration applications (e.g., vacuum cleaner bags).
In addition, in U.S. Pat. No. 5,023,130 (Simpson et al.) hydroentangled flash spun polyolefin webs are disclosed having enhanced softness and barrier properties. Spunlaced fabrics made according to the Simpson et al. process are commercially available from E.I. du Pont de Nemours and Company under the trademarks "TYPRO HC" and "ComforMax IB". These spunlaced fabrics have proven useful as inner-layer barrier liners for garments, sleeping bags, comforters and pillows. However, although these spunlaced fabrics work well with fiber insulating or filling materials, they do not work well in preventing perculation when duck feathers or down filling are used. Experience has shown that feather quills migrate through available openings in the fabric.
Moreover, in U.S. Pat. No. 4,681,801 (Eian et al.) melt-blown fiber webs are hydroentangled to force reinforcing fibers therethrough. The reinforcing fibers are hydraulically needled, preferably mechanically needled, through the melt-blown fibers and then bonded to fibers on the opposing faces of the layer of melt-blown fibers to hold the reinforcing fibers in position.
Clearly, what is needed is a process and spunlaced nonwoven fabric which do not have the deficiencies inherent in the prior art. In particular, in the unfused state, the spunlaced fabric should have bulk and should prevent perculation of feathers or down filling when used as a barrier liner in garments, pillows or the like. In addition, in the thermally bonded state, the spunlaced fabric should provide an increased level of permeability such that it is useful in sensitive microfiltration applications. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.